refractor

The instrumented 120mm-APO Refractor.

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The main optical instrument at the "Kolkert-Ob= servatory" is an instrumented "go to"-apochromatic refractor. The following pictures give a good impression of the telesc= ope in its present state.

Besides a description of the technical specificati= ons of the instrumented 120mm-APO, results are pres= ented of diffraction patterns (ie. star-tests) recorded with both the original achromatic and t= he modified apochromatic telescope. Furthermore pictures of planets are shown which were made with the a= pochromatic set-up.

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Technical specifications of the APO-modified 120mm refractor of = Synta.<= o:p>

I. General/Original.

The original outfit of the 120mm Synta-achromate OTA consists of a Fraunhofer doublet objective moun= ted in an adjustable lens cell being part of a baffled aluminium tube. The 2" focuser is of the rack-and pinion type and contains an aluminium draw-tube. This OTA is mounted on the CG-4 equatorial mount of my other refractor, the C102-HD.

Two standard eyepieces are available, a 7.6 mm Plössl= , series 3000 of Meade and a 20 mm one of Celestron. A Celestron polar axis finder scope with cross wire illumination is available, too. The visual finder telescope is a 6x30 Celestron type with cross wire.=

The focal length of the 120 mm achromat is standard= 1000 mm ( F/8.3), the light gathering power is 293, t= he max. magnitude at which objects can be observed = is 13.1 and the optical resolution (Dawes limit) is 0.96" (arc).

II. Astrophotography.

Images of sky-objects are photographed with a PC-controlled, 1-stage cooled (-30°C) CCD-astrocamera, the 12 bits Starli= ght-Xpress MX5-C, which makes it possible to record repetitively single-shot multi-colour pictures. Monitoring and control is performed with the program STAR2000/MX5-C and/or ASTROART2.0 with the MX5-C plug-in. Raw images of exposures are colour-converted to LRGB images and processed with ASTROART, too.

III. Optical aids and Sky-data.

Optical options have been extended by replacing the 6x30 finder scope with a 9x50 o= ne and fitting a 0.1 lux CCD-= videocamera at prime focus to it. This greatly enhances finding and focusing objects up= to Magnitude 7. The 0.33 inch B/W sensor of the 12V camera has 725(H)x528(V) pixels, gives 380 TV lines in horizontal and 450 lines in vertical directio= n, has an automatic iris and shutter speed (1/50 to 1/32000), has a 2:1 interl= ace scanning system and a S/N ratio better than 50dB.

The real-time image of this combination is depicted on a TV-monitor on which sc= reen a centering cross has been drawn. The circular = FOV of this combination is 2.4° and is, together with the square FOV of the single-shot CCD-astrocamera projected at the Ea= rth Centered Universe(ECU)-scr= een, showing the relevant part of the sky. The centers of both FOV's are made to coincide at the center cross of the TV monitor and the center of the CCD-chip of the astrocamera. This simpli= fies finding and centering sky-objects to a great ex= tent. General information on sky-objects is retrieved from the well-documented REDSHIFT-3 code.

Further on a telecompressor lens (Meade 0.63x), a Televue Powermate 5x Barl= ow and a 12 mm Kellner eyepiece with illuminated double = cross wire, mounted in a Flip Mirror Finder (True Technologies) equipped with an = IR-filter, have been added. For testing optimized visual focusing and determination of spherical aberration of the optical train, a dummy eyepiece with a 7 lines = per mm Ronchi grating is available. Maximum F-numbe= r to be obtained with the Televue Powermate is F/41.6 at a focal length of 5000 mm.

IV. Modifications.

A. Electromechanical.

The CG-4 mount, of which the bearings and drives have been re-adjusted to satisfaction, has been upgraded with:

1. an autonomous controlled stepper motor for the right ascension axis (RA), max. 8x sidereal speed, and

2. a variable speed DC declination motor (JIM's Mobile)= to set/control the declination.

The OTA focusing device has been equipped with a variable speed DC focusing motor (= JIM's Mo= bile) to focus the image plane and a digital focus read-out (DFRO) to determine i= ts position.

The slew rates of the motorized mount are modest(2&d= eg;/minute max.), but suffice at the moment. Slewing to an object chosen in the real t= ime planetarium and telescope control program ECU, gives the following pointing accuracy: s RA=3D ± 3s and s DEC=3D ± 49"(a= rc).

The backlash in the two drives of the mount for RA- and DEC rotation at reversa= l is 6.6s and 51"(arc), respectively. Hysteresis is well within these values.

The rack-and-pinion focuser has been exchanged with a dual speed (1:10) Crayford one from Willi= ams Optics. It has been adapted for motorized drive and digital position read-o= ut.

Focusing on an object is done automatically with the motorized Crayford device. The = resolution of the DFRO ( see below), and so the accuracy to determine the focus plane with this setup is 0.87°rotation of= the axis of the focuser, which equals 0.059 mm axial displacement. At a 1/4 l defocusing aberration, = the depth of focus is allowed to be within +1/8 and -1/8l. Then, delta-focus runs= from 0.0597 mm for F/8.3 at l =3D0.43µm to 2.284 mm for F/42 at l =3D0.66µm, respec= tively. So, for the F/42-set-up and a resolution of ± 0.059 mm axial displacement for one count of the DFRO (see below), the defocusing aberrati= on should be better than 1/64 l ! =

All accessories to be used at the focuser draw-tube end are fitted with three setting screws to facilitate alignment of the optical train, for which a Cheshire eyepiece = and a TL-light source are used.

B. Optical conversion = from achromate to apochromate;= the CHROMACORR.

A newly developed item which is a leap-ahead in the reduction of the secondary spec= trum and spherical aberration, i.e. the 150mm achro-apo converter or Chromacorr, developed at ARIES Ins= truments, is also installed. This three-lens system has an over-correction in spheric= al aberration of +1/7λ and was ch= osen as the doublet objective of the 120 mm shows an under-correction of about –1/7λ<= b>. It is the Chromacorr-O1 and it is threaded in the forward-end of a 2-inch outer diame= ter extension tube with such a length that the distance in between the focal st= op of an ocular or Barlow and the thread of the Chromacor= r can be set from 190 mm up to 225 mm. This assembly is now centered in the draw-tube with the help of a compression ring. The Chromacorr extension tube centered in the draw-tube of the Crayford focuser and the 9x50 finder scope is shown below.

Both for the original set-up and for the optical train extended with the Chromacorr, results of star-testing, Ronchi-testing and secondary spectrum determination up to F/42 are reported here.

First results of star images are also presented here. For the white star Altair, images (3 sec. exposure) were made at F/8.3 up to F/42 with and without the Chromacorr-O1 installed. Optimal collimation is mandatory in these test and asks for some praxis.

V. Automatization.

A. Digital Setting Circles and Focus Read-Out. =

The motorized items are connected to digital setting circles (DSC's) and to a d= igital focus read out (DFRO) for which encoders have been connected directly to the different rotation axes of the equatorial mount and to the focusing device. These systems are home-made. Digitizing of the different encoder signals oc= curs with the PCB of a modified PS2-mouse-now called the "Astro-mouse"-and are converted into discrete counts with the soft= ware program "encoder" which runs on an old 364 PC. With a laplink, that PC is connected to the main PC which ru= ns the real time planetarium and telescope control program Earth Centered Universe (ECU). The counts of the DSC's are transformed into RA-and DEC-val= ues which are presented both as a (moving) cursor and a numerical window on the PC/ECU-screen which depicts the area of the sky the telescope is pointing a= t. The position of the focus plane is read-out on the 364 PC as integers with a respective sign for inter- and intra-focus.

B. Telescope control.

1. RA- and DEC-motion.

Actuation of both the DC DEC-motor and RA-stepper motor is being executed via the STAR2000 interface module in combination with the accompanying Relay Box. T= he speed of the DEC-motor can be continuously controlled manually, while their= are next to sidereal rate, two(2) speed options for = the RA-motor; 2x and 8x sidereal speed in both East- and West-direction. The lo= wer speeds are used for self-guiding and tracking of a chosen celestial object = in the FOV of the telescope, while the higher speeds are used for slewing purposes.

The STAR2000 interface module from Starlight-Xpress= is operated by either the STAR2000 software (telescope control panel) of the M= X5-C astrocamera or by the telescope control window = (move panel) of ASTROART2.0.

Finding an object, slewing to an object and centering i= t is performed by combining the telescope control panel = window with the real-time local sky-chart of ECU. Choosing an object in ECU gives = its co-ordinates and deviation of the present position the telescope is pointing at. The latter information is coming from the DSC’s on the equatorial mount and their "Astro-mouse" interfa= ce with ECU. By activating the relevant buttons in the control/move panel, the deviations are zeroed and the chosen object arrives near or at the optical = axis of the telescope as observed in the continuously updated image field of the= astrocamera MX5-C. For the latter situation , the camera operates in the focus-mode under ASTROART2.0. At that moment the position of the optical axis of the telescope and the position of the chosen object are synchronized and centered= in ECU.

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2. Focusing.

Focusing is performed with the motorized Crayford focuser and the DFRO. The focus-mo= tor is actuated via the North- and South-buttons of the move-panel of the teles= cope control window in STAR2000 and the result is real-time observed in the regularly updated image field of the "focus"-window in ASTROART2.= 0.

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C. Tracking and Guiding.

During tracking or self-guiding of an object, a real time ECU-window shows the possible deviation in RA- and DEC-values of the actual telescope position (= i.e. pointing position of it's optical axis) from the coordinates of the chosen object. Corrections can be performed both manuall= y by actuating the hand controllers of the RA- and/or DEC motors and automatical= ly by using the "guide"-option of ASTROART. In that case an image wi= ndow shows the initial and actual position of the guide object and the STAR2000-protocol derives correction commands from the deviations of the ac= tual position and actuates the motors to correct the position. The guiding information is updated every 0.2 seconds or at longer intervals. Control is within ±1 pixel (about 0.6"at F/40) of the CCD-chip of the astrocamera for an indefinitely period of time.<= /o:p>

For long exposures, accurate control of the position of the object to be studie= d on the CCD-chip of the astrocamera is also done wi= th the automatic self-guide option in ASTROART2.0. Control can be obtained to the 1 pixel level for an adequately picked guide-star in the FOV of the camera, w= hich suppresses trailing on the image, completely.

Raw MX5-C images at F/40 of Upsilon-29. Left: 15 m= inutes exposure without self-guide. Right: 15 minutes exposure with ASTROART2.0 automatic self-guide.

VI. Numerical values of the set-up.

A. FOV (Field of V= iew).

The calculated FOV of the ICX 055CK chip of the Starlight Express CCD-camera wi= th chip-dimensions of 4.9(horizontal) x 3.6(vertical) mm is for different F-numbers:

1. F/5,3 (0.63 telecompressor),<= span style=3D'color:yellow'>--- -f=3D 630 mm-------26.8' x 19.= 7' (arc)

2. F/8.3 (standard),--= ----------------- -f=3D 1000 m= m-----16.9' x 12.4' (arc)

3. F/16.6 (one Barlow, 2x),--= -------- f=3D 2000 mm---= --- 8.4' x 6.2' (arc)

4. F/33.2 (two Barlow's, each 2x),--f=3D 4= 000 mm--- ---4.2' x 3.1' (arc)

5. F/41.6 (Televue Powermate<= /span> 5x), f=3D 5000 mm-------3.4' x 2.5' (ar= c)

For instance for Jupiter, this leads to a 32% PC-screen occupancy at F/42.

For the sky-occupancy per pixel in the vertical direction of the chip of this camer= a, which has 145.000 pixels (500 hor. x 290 vert.), the following values are obtained:=

1. F/5.3----------- -4.1"(arc)

2. F/8.3---------- --2.6"(arc)

3. F/16.6-----------1.3"(arc)

4. F/33.2-----------0.6"(arc)

5. F/41.6 ----------0.5"(arc)

B. Optical Resolut= ion.

The power to resolve individual point sources at optical wavelengths from each other = for this optical system in combination with the camera mentioned, has been dedu= ced, too. The dimensions of the pixels are 9.8 x 12.6 mm. For the average pixel= size of 11.2 m= m and at the maximum response wavelength (l =3D 0.56 mm) of the chip, the resolution per pixel at the different F-values is;

1. resolution per pixel at F/8.3 -----------= -2.31" = (arc)

2. resolution per pixel at F/16.6-----------= 1.15" = (arc)

3. resolution per pixel at F/33.2-----------= 0.58" = (arc)

4. resolution per pixel at F/41.6---------- 0.46" (arc)

The value under 4 is the so-called Nyquist-criterio= n for correctly sampling the image of the observed object, for which the Airy-disk diameter is 0.56 x F-value (in mm). Then good results are obtained with image-processing.

As it appears from experimentation, the chromatic aberration at F/42 is negligible (secondary spectrum is about 0.00006 times the focal length f). =

The optimal F-number for observing planets for this system is F/40 and for deep= -sky objects it is F/20. The magnification at F/42 as seen on my PC-screen, is 790x.

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STAR-TESTING of the modified 1= 20 mm APO-refractor.

I. Without the Chromacorr installed.

A. Diffract= ion patterns for Altair at full aperture.

inside focus---------------= --------------------------------------------focus----------------------------------------------------= --------outside focus

These results of diffraction rings for the white star Altair (magnitude +0.79) of= the 120 mm doublet objective (f=3D1000 mm) of the Synta refractor (as bought) were obtained with the Starlight MX-5C astrocamera equipped with an IR-filter. The magnifica= tion was about 158 x.

B. Summary of resu= lts.

-The secondary spectrum (i.e. difference in focus position D f between yellowish and purple image) at F/8.3 is .62 mm; so D f/f ≈= 0.00062 fo= r this optical system. Quality-Fraunhofer achromatic doublets have a corresponding value of about 0.0005. =

-Comparing diffraction patterns with results of simulations (Aberration code) leads to= the conclusion that the spherical aberration of the doublet objective without <= span class=3DSpellE>Chromacorr is –0.14 l .<= span lang=3DEN-GB style=3D'font-size:10.0pt;mso-ansi-language:EN-GB'>=

-Some astigmatism and an S-zone at 30%-radius is present.
-Ronchi-testing showed straight, sharp and parallel li= nes and so is to unsensitive for deducing aberratio= ns in this case.

II. With the Chromacorr O-1 installed.

A. Diffraction patterns for Altair.

1. Stopped-down= aperture.

After collimating the two elements of the objective with the optical axis of the OTA-tube with the help of the adjustable lens cell and the Cheshire eyepiece, the optical train involving the Chromacorr O-1 and the doublet objective was aligned.

These are the diffraction rings for Altair, obtained from star-testing with the modified 120 mm Synta doublet refractor stopped= -down to a 50 mm aperture and equipped with the Chromacorr-O1 plus self-centering draw-tube adapter. The images were recorded= with the MX-5C astrocamera plus IR-filter. The atmos= phere was rather steady during these tests.

-Summary of result= s.

- Astigmatism has been removed by loosening the retaining ring pressing the rubber O-ring onto the doublet objective in its cell.

-The secondary spectrum Δf was improv= ed to less than 0.059 mm (the resolution of the Digital Focus Read-Out, DFRO) and= the spherical undercorrection was estimated to be l= ess than 1/42= l. Both para= meters have been improved substantially by using the Chromacorr-O1! The magnificat= ion was 158x.

-It is noticed that Δf/f=3D0.000= 059, indicating that this 120mm OTA plus Chromacorr = O-1 behaves as an excellent apochromate!=

-In the stopped-down mode, not much can be seen of the S-zone. So it's amplitude must be smaller than 1/8λ.

-Ronchi-testing showed sharp, straight and parallel li= nes all over the field of view.

2. Full = aperture.

Besides aperture stopped-down testing, star-testing was also performed with the full aperture of the Synta 120 mm refractor equipped= with the Chromacorr O-1.

The images above show the diffraction rings for Altair as recorded with the MX-5C astrocamera.

The effects of the S-zone are still visible. Its effect on images of planets wi= ll determine how severe this zoning is. Again the magnification was abou= t 158x.

B. Another star.

These are the diffraction rings for a magnitude +2 light-yellow star of the 120 m= m Synta doublet refractor equipped with the Chromacorr-= O1. The images were recorded with the MX-5C astrocamera. The atmosphere was rather steady and the magnification was about 158x.

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C. Effect of Vixen= Deluxe Barlow-lenses on optical performance in star-testing.

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These are the diffraction rings for Altair of the 120 mm Synta doublet refractor plus Chromacorr-O1 and two Vixen Deluxe Barlow’s (t= wo times each) as recorded with the MX-5C astrocamera. The Chromacorr was not optimally aligned, i.e. = there is a little lateral red-blue shift.

A minor effect of the S-zone can be seen in these images while some optical pinchin= g of to tightly-fastened set-screws of the Barlow-lenses can be seen, too.<= /o:p>

The magnification is 632x.

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D. The effect of a= Televue Powermate 5x Barl= ow on optical performance.

These are the diff= raction rings for Rigel, a magnitude 0.18 blue star, of the 120 mm Synta = doublet refractor stopped-down to 50 mm. The telescope was equipped with the Chromacorr-O1 and the Televue powermate 5x. The images were taken with the MX5-C astrocamera including an IR-filter.=

The atmosphere was= rather steady and the magnification was 790x.

The magnitude of t= he S-zone (smaller than 1/8λ) is too small to be of importance for image degradation as can be s= een in the image of Saturnus(see under III).

III. Saturnus and Jupiter.

These multi-shot c= omposed pictures were taken with the 120 mm Synta doubl= et refractor, equipped with the Chromacorr-O1 and the Tel= evue powermate 5x. The magnification was 790x, while the outdoor temper= ature was -5° C.

IV<= span style=3D'font-size:13.5pt'>. Mars.

This colour-conver= ted single-shot picture was taken at 23.46 hour on 4 September 2003 with the MX= 5-C camera. The 120 mm Synta doublet refractor was equipped with theChromacorr-O1 and Televue Powermate 5x.

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14 September 2006 =